Since lithium ion secondary batteries can achieve high energy densities among devices such as lithium ion secondary batteries and capacitors, these attract attention as power sources for cell phones and laptop computers, and also as large power sources for electricity storage and power sources for automobiles.
Although lithium ion secondary batteries can achieve high energy densities, up-sizing makes the energy density gigantic, and higher safety is demanded. For example, in large power sources for electricity storage and power sources for automobiles, especially high safety is demanded, and as safety measures, there are applied the structural design of cells, packages and the like, protection circuits, electrode materials, additives having an overcharge protection function, the reinforcement of shutdown function of separators, and the like.
Lithium ion secondary batteries use aprotic solvents such as cyclic carbonates and chain carbonates as electrolyte solvents; and these carbonates are characterized by having a low flash point and being combustible though having a high dielectric constant and a high ionic conductivity.
One means of further enhancing the safety of lithium ion secondary batteries is making electrolyte solutions flame-retardant. As a technique for making electrolyte solutions flame-retardant, methods of adding a phosphazene compound as a flame retardant are disclosed.
For example, a nonaqueous electrolyte battery of Patent Literature 1 uses a solution in which a lithium salt is dissolved in a phosphazene derivative as the electrolyte, or a solution in which a lithium salt is dissolved in a solvent in which an aprotic organic solvent is further added in a phosphazene derivative. It is disclosed that thereby there arises no danger such as burst and ignition even in abnormal cases such as short circuits and that the excellent battery performance can be achieved.
A nonaqueous electrolyte battery of Patent Literature 2 uses a solution in which a lithium salt is dissolved in a chain-type phosphazene derivative as the electrolyte, or a solution in which a lithium salt is dissolved in a solvent in which an aprotic organic solvent is further added in a phosphazene derivative. It is disclosed that thereby there arises no danger such as burst and ignition even in abnormal cases such as short circuits and that the excellent battery characteristics can be achieved.
Patent Literature 3 describes a nonaqueous-type electrolyte solution secondary battery which has a positive electrode, a negative electrode, and a nonaqueous-type electrolyte solution containing a supporting salt, an organic solvent and a phosphazene derivative, wherein the potential window of the phosphazene derivative is in the range of a lower-limit value of +0.5 V or lower and an upper-limit value of +4.5 V or higher, and the potential window of the organic solvent is in a broader range than that of the phosphazene derivative.
Patent Literature 4 describes a nonaqueous-type electrolyte solution secondary battery which has a positive electrode, a negative electrode, and a nonaqueous-type electrolyte solution containing a supporting salt and a phosphazene derivative whose lithium salt solution (0.5 mol/l) has a conductivity of at least 2.0 mS/cm.
A technology is known which uses, as an additive, a substance reductively degraded at a higher potential than those of carbonates used as electrolyte solution solvents and forming an SEI (Solid Electrolyte Interface) being a protection membrane having a high lithium ion permeability. It is known that since the SEI has large effects on the charge/discharge efficiency, the cycle characteristics and the safety, control of the SEI at a negative electrode is essential, and the irreversible capacity of carbon materials and oxide materials can be reduced by the SEI.
Patent Literature 5 describes provision of an excellent nonaqueous-type electrolyte solution containing a lithium salt and a nonaqueous solvent, which can secure the safety and reliability in abnormal heating and the like of a battery and can also provide good battery performance such as cycle characteristics by further incorporation of a cyclic carbonate ester having a carbon-carbon unsaturated bond in the molecule and 1% by mass or more and 25% by mass or less of a phosphazene derivative based on the nonaqueous-type electrolyte solution.
A nonaqueous-type electrolyte solution for a battery disclosed in Patent Literature 6 contains a nonaqueous solution containing a cyclic phosphazene compound and a difluorophosphate ester compound, and at least one cyclic sulfur compound selected from the group consisting of 1,3-propanesultone, 1,3-butanesultone, 1,4-butanesultone and 1,3,2-dioxathiolane-2,2-dioxide, and a supporting salt. It is disclosed that the excellent battery performance and high safety even in a high-temperature environment are thereby imparted to the battery.